Add REDIS_MIN_RESERVED_FDS define for open fds

Also update the original REDIS_EVENTLOOP_FDSET_INCR to
include REDIS_MIN_RESERVED_FDS. REDIS_EVENTLOOP_FDSET_INCR
exists to make sure more than (maxclients+RESERVED) entries
are allocated, but we can only guarantee that if we include
the current value of REDIS_MIN_RESERVED_FDS as a minimum
for the INCR size.

src/redis.c

@@ -1491,20 +1491,20 @@ void initServerConfig() {
 
 /* This function will try to raise the max number of open files accordingly to
  * the configured max number of clients. It also reserves a number of file
- * descriptors (REDIS_EVENTLOOP_FDSET_INCR) for extra operations of
+ * descriptors (REDIS_MIN_RESERVED_FDS) for extra operations of
  * persistence, listening sockets, log files and so forth.
  *
  * If it will not be possible to set the limit accordingly to the configured
  * max number of clients, the function will do the reverse setting
  * server.maxclients to the value that we can actually handle. */
 void adjustOpenFilesLimit(void) {
-    rlim_t maxfiles = server.maxclients+REDIS_EVENTLOOP_FDSET_INCR;
+    rlim_t maxfiles = server.maxclients+REDIS_MIN_RESERVED_FDS;
     struct rlimit limit;
 
     if (getrlimit(RLIMIT_NOFILE,&limit) == -1) {
         redisLog(REDIS_WARNING,"Unable to obtain the current NOFILE limit (%s), assuming 1024 and setting the max clients configuration accordingly.",
             strerror(errno));
-        server.maxclients = 1024-REDIS_EVENTLOOP_FDSET_INCR;
+        server.maxclients = 1024-REDIS_MIN_RESERVED_FDS;
     } else {
         rlim_t oldlimit = limit.rlim_cur;
 
@@ -1518,7 +1518,7 @@ void adjustOpenFilesLimit(void) {
                 limit.rlim_cur = f;
                 limit.rlim_max = f;
                 if (setrlimit(RLIMIT_NOFILE,&limit) != -1) break;
-                f -= REDIS_EVENTLOOP_FDSET_INCR;
+                f -= REDIS_MIN_RESERVED_FDS;
                 if (f > limit.rlim_cur) {
                     /* Instead of getting smaller, f just got bigger.
                      * That means it wrapped around its unsigned floor
@@ -1534,7 +1534,7 @@ void adjustOpenFilesLimit(void) {
             if (f < oldlimit) f = oldlimit;
             if (f != maxfiles) {
                 int old_maxclients = server.maxclients;
-                server.maxclients = f-REDIS_EVENTLOOP_FDSET_INCR;
+                server.maxclients = f-REDIS_MIN_RESERVED_FDS;
                 if (server.maxclients < 1) {
                     redisLog(REDIS_WARNING,"Your current 'ulimit -n' "
                         "of %llu is not enough for Redis to start. "

src/redis.h

@@ -123,6 +123,7 @@
 #define REDIS_IP_STR_LEN INET6_ADDRSTRLEN
 #define REDIS_PEER_ID_LEN (REDIS_IP_STR_LEN+32) /* Must be enough for ip:port */
 #define REDIS_BINDADDR_MAX 16
+#define REDIS_MIN_RESERVED_FDS 32
 
 #define ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP 20 /* Loopkups per loop. */
 #define ACTIVE_EXPIRE_CYCLE_FAST_DURATION 1000 /* Microseconds */
@@ -139,9 +140,9 @@
 #define REDIS_LONGSTR_SIZE      21          /* Bytes needed for long -> str */
 #define REDIS_AOF_AUTOSYNC_BYTES (1024*1024*32) /* fdatasync every 32MB */
 /* When configuring the Redis eventloop, we setup it so that the total number
- * of file descriptors we can handle are server.maxclients + FDSET_INCR
+ * of file descriptors we can handle are server.maxclients + RESERVED_FDS + FDSET_INCR
  * that is our safety margin. */
-#define REDIS_EVENTLOOP_FDSET_INCR 128
+#define REDIS_EVENTLOOP_FDSET_INCR (REDIS_MIN_RESERVED_FDS+96)
 
 /* Hash table parameters */
 #define REDIS_HT_MINFILL        10      /* Minimal hash table fill 10% */